System.Text.StringBuilder strTest=new System.Text.StringBuilder();Strings are immutable. immutable means every time we alterthe string a new object is created. hence lower the performance. while stringBuilder are mutable. so it increase the performancewhere we need to perform altered, insert and remove operations. But it is not recommended to use StringBuilder always. for small string where you need to perform less operation then use stringand in case of large string and more operation use StringBuilder.